This invention relates to production apparatus having multi-impression dies for vacuum forming articles in sheet plastic.
In such apparatus, the many die forms are detachably fixed in a die platten. It is the conventional practice to provide a stripper plate to peel the plastic sheet off the die forms after the forming operation. The stripper plate has holes in it corresponding to the die-forms, and it is arranged that the plastic sheet is sucked, by the vacuum, against the die-forms through the holes in the stripper plate, but that the rest of the sheet, i.e., that portion of the sheet between the die-forms, lies against the stripper plate. After forming has taken place, the stripper plate is moved some small distance away from the die platten; the sheet of plastic moves with it, causing the formed areas to release from their respective die-forms.
Vacuum-formed plastic articles often have a lip comprising a portion of their form disposed in a direction substantially at right angles to the plane of the web of material from which the article was made. A lid for a disposable drinking cup is a typical example of such an article, in that such a lid has a lip around its periphery, which in some cases is actually slightly re-entrant with respect to the web. The re-entrant shape is important in providing a positive snap when the lid is fitted to the cup.
Such a lip, especially when it is re-entrant, means that there can be some difficulty in stripping the web of moulded articles from the die-forms. It is important to apply the stripping force as closely as possible to the edge of the article. If there is a land of any appreciable size between the lip and where the stripping force is applied, then the material will tend to flex in that land, which could cause the formed shape to turn inside out.
It is desirable therefore, from the point of view of an efficient and reliable stripping operation, to see that the edge of the die-form clearance holes in the stripper plate lie as closely as possible to the lip or edge of the formed shape.
When the articles are always the same size, then it is a simple matter to provide holes in the stripper plate of such a diameter that the edges of the holes are the desired small distance from the lip of the article. When, however, it is required to use the same vacuum forming apparatus to produce articles in batches that are sometimes one size, sometimes another, then a separate stripper plate for each batch would be needed. When one considers also that not all the die-forms of a multi-impression die platten might need to be changed from batch-to-batch, one can see that the number of stripper plates needed would be quite uneconomical.
Thus, there are three alternatives when carrying out vacuum forming in batches: first, the production engineer might provide large holes in the stripper plate, which would allow various sizes of die-form to be used but would means that only on the largest of the sizes could the stripping be regarded as a reliable operation; second, the production engineer might decide to provide a large number of stripper plates, each with a set of holes that correspond very closely with a particular set-up or arrangement of die-forms, but now he is faced with the enormous cost of making all the stripper plates, and of changing them over between batches; or, thirdly, he might decide to use spacers.
With the use of spacers, only one stripper plate need be provided, and this plate has holes to cater for the largest dieforms. When the plate is used with smaller die-forms, a suitable spacer is inserted to bring the edge of the hole in the stripper plate in effect closer to the edge of the formed shape.
A problem with the use of spacers is that the variation in hole size is quite small. Again taking the example of disposable drinking cup lids, the difference in diameter lid-to-lid might be 1 mm; it would be a problem to secure a spacer to the holes in the stripper plate, which has a radial thickness of only 0.5 mm, particularly since the die-forms are packed as tightly as possible so that the hole-to-hole spacing is very small. Furthermore, the spacers should be assemble-able from one side of the stripper plate without access at all to the other side of the stripper plate, if the spacers are to be changed with the plate in situ.
For these reasons, the conventional methods of securing spacers to the stripper plate, such as bolting them in, turn out to have great drawbacks to the economics of bath production. It is an aim of the present invention to provide a manner of constructing and arranging the spacers which allows batch production to be economically achieved. Another requirement is that the spacer must effectively be in the form of a continuous ring, i.e., there can be no gaps since, if there were, they would act like the wide lands described above. It is assumed for the purpose of description that the die-form, and hence the spacer, is circular, but other shapes are possible.